The present invention relates to the purification of mercaptobenzothiazole, and more particularly, it relates to methods for purifying crude mercaptobenzothiazole obtained by synthetic processes.
2-Mercaptobenzothiazole is commercially utilized in the elastomer conversion as a vulcanization accelerator. Most importantly, it is a premier starting material for the synthesis of more complex vulcanization accelerators. These accelerators act both to delay the scorch time of the mixtures to be vulcanized and permit choosing the best vulcanization parameters for the preparation of a particular article. It is also employed for the synthesis of plant-protection or pharmaceutical compounds and is also included as an anti-corrosive agent in metal protection preparations.
Most of the known processes of manufacture, which are basically derived from U.S. Pat. No. 1,631,871, rely on the reaction of aniline, sulfur and carbon disulfide at high temperature and high pressure. Others rely either on the reaction of thiocarbanilide, carbon disulfide and sulfur (U.S. Pat. No. 1,712,968), or on the reaction of orthochloronitrobenzene, hydrogen sulfide or an alkali metal sulfide, and carbon disulfide (U.S. Pat. No. 1,960,205). Alternatively, German Pat. No. 2,551,060 uses the reaction of benzothiazole and sulfur.
The reaction products obtained from these various processes cannot be employed as such. The products so produced contain unreacted starting materials, for example, aniline, and they also contain by-products and intermediates such as benzothiazole and anilinobenzothiazole. Careful purification of the crude reaction products is required.
Numerous purification processes have been proposed so far and are the subject of U.S. Pat. Nos. 1,631,871; 2,090,233; 2,658,864; 2,730,528; 3,030,373; 3,031,073; and 3,818,025, and French Pat. Nos. 2,135,807 and 2,397,409. A discussion of these methods is at pages 2 to 4 of French Pat. No. 2,450,828 which discusses their respective disadvantages and suggests another purification technique which involves treating the crude synthesis product with aniline, the starting material of the reaction.
According to this technique, aniline can be added to the crude product either directly in the synthesis reactor, before release of the pressure to atmospheric pressure, at a temperature preferably in the range 180.degree.-220.degree. C., or, after release of the pressure to atmospheric pressure, at a temperature between ambient temperature and the boiling point of aniline, that is to say, in the range from 15.degree. to 184.degree. C. The mercaptobenzothiazole, so rendered insoluble in aniline, is filtered off after cooling to ambient temperature and is then washed with aniline. The aniline present in the filter cake can be removed, for example, by steam distillation or by vacuum evaporation. Because of the solubility of mercaptobenzothiazole in aniline, it is essential for an economical process that the organic phases originating from the filtration and the washing of the crystallized product be recycled.
The process of French Pat. No. 2,450,828 represents a very clear improvement relative to the prior art processes, because it makes it possible readily to recover the unreacted starting materials and the useable by-products which may be present in the reaction product (in particular, the benzothiazole). The recovery is facilitated because materials other than the starting materials for the reaction are used. This process virtually eliminates the problems posed by the treatment of aqueous wastes from conventional industrial plants.
However, the disadvantage of the process according to French Pat. No. 2,450,828 lies in the high aniline content which must be recovered from the product isolated at ambient temperature. Since this quantity is of the order of 40 to 50%, the unfavorable heat balance of this recovery severely compromises the advantages of the process.